Linear Algebra for Computer Vision - part 2

CMSC 828 D

Outline• Background and potpourri• Summation Convention• Eigenvalues and Eigenvectors• Rank and Degeneracy• Gram Schmidt Orthogonalization• Fredholm Alternative Theorem• Least Squares Formulation• Singular Value Decomposition• Applications

Summary: Linear Spaces• n dimensional points in a vector space.

– Length, distance, angles– Dot product (inner product)

• Linear dependence of a set of vectors• Basis : a collection of n independent vectors so

that any vector can be expressed as a sum of these vectors

• Orthogonality <a,b>=0• Orthogonal basis: basis vectors satisfy <bi,bj> = 0• Vector is represented in a particular basis

(coordinate system) <u,bi> = ui

• Linear Manifolds (M): linear spaces that are subsets of the space that are closed under vector addition and scalar multiplication– If vectors u and v belong to the manifold then

so do α1 u + α2v– Manifold must contain zero vector– Essentially a full linear space of

smaller dimension.

• Span of a set of vectors: set of allvectors that can be created by scalar multiplication and addition.

• Vectors in the space that are in the rest of the space are orthogonal to vectors in M. (M⊥ )

• Projection Theorem: any vector in the space X can be written only one way in terms of a vector in M and a vector in M⊥.

Ou

2u

-u

v

Manifold spanned by u

Gram Schmidt Orthogonalization• Given a set of basis vectors (b1,b2, …, bn) construct

an orthonormal basis (e1,e2, …, en) from it.– Set e1= b1/|| b1||– g2= b2 − < b2, e1> e1, e2 = g2/||g2||– For k=3, …,n

gk= bk −Σj< bk, ej> ej, ek = gk/||gk||

Euclidean 3D• Three directions with basis vectors i,j,k or

e1,e2,e3, with ei .ej= δij• Distance between two vectors u and v is ||u-v||• Dot product of two vectors u and v is ||u||.||v|| cos θ• Cross product of two vectors is u×v

– magnitude equal to the area of the parallelogram formed by u and v.

– Magnitude is ||u|| ||v|| sin θ– Direction is perpendicular to u and v

so that the three vectors form a right handed system• Is also written using the permutation symbol εijk

1 2 3

1 2 3

= u u uv v v

×i j k

u v

Summation Convention• Boldface, transpose symbol and summation signs are

tiresome. – Especially if you have to do things such as differentiation

• Vectors can be written in terms of unit basis vectorsa=a1i+a2j+a3k= a1e1+a2e2+a3e3

• However, even this is clumsy. E.g., in 10 dimensionsa= a1e1+a2e2+…+a10e10 =Σi=1

10 aiei

• Notice that index i occurs twice in the expression.– Einstein noticed this always occurred, so whenever index was

repeated twice he avoided writing Σi

– instead of writing Σiaibi, write aibi with the Σi implied

Permutation Symbol• Permutation symbol. εijk

– If i, j and k are in cyclic order εijk =1 • Cyclic => (1,2,3) or (2,3,1) or (3,1,2)

– If in anticyclic order εijk =-1• Anticyclic => (3,2,1) or (2,1,3) or (1,3,2)

– Else, εijk =0• (1,1,2), (2,3,3), …

• c = a×b => ci = εijkajbk• ε δ identity εijk εirs = δjr δks- δjs δkr

– Very useful in proving vector identities• Indicial notation is also essential for working with tensors

– Tensors are essentially linear operators (matrices or their generalizations to higher dimensions)

Examples• Ai Bi in 2 dimensions: A1B1+A2B2

• Aij Bjk in 3D? We have 3 indices here (i,j,k), but only j is repeated twice and so it is Ai1B1k + Ai2B2k+Ai3B3k

• Matrix vector product Ax = Aijxj, Atx = Aijxi

• (a × b). c= εijkajbkci– Using indicial notation can easily show

a .(b×c) = (a × b). c• Homework: show a ×(b ×c)=b(a.c)-c(a.b)

Operators / Matrices• Linear Operator A(α1 u + α2v) = α1 Au + α2 Av• maps one vector to another

Ax=b– m×n dimensional matrix A multiplying a n dimensional vector

x to produce a m dimensional vector b in the dual space• Square matrix of dimension n by n takes vector to

another vector in the same space.• Matrix entries are representations of the matrix using

basis vectors Aij = < Abj,bi>• Eigenvectors are characteristic directions of the matrix.• Matrix decomposition is a factorization of a matrix into

matrices with specific properties.

Rank and Null Space• Range of a m×n dimensional matrix A

Range (A) = {y∈!m: y=Ax for some x ∈!n}• Null space of A is the set of vectors which it takes to zero.

Null(A) = {x ∈!n : Ax = 0}• Rank of a matrix is the dimension of its range.

Rank (A) = Rank (At)– Maximal number of independent rows or columns

• Dimension of Null(A)+Rank(A) = n

Norm of a matrix• ||A|| ≥0 ||Ax||≤||A|| ||x|| • ||A||F =[aij aij]1/2 Froebenius norm. If A is diagonal

||A||F=[a112+ a22

2+…+ ann2 ]1/2

• ||A||2 = maxx ||Ax||2/||x||2. Can show 2 norm = square root of largest eigenvalue of AtA

Orthogonality• Two vectors are orthogonal if <u,v>=0• Orthogonal matrix is composed of

orthogonal vectors as columns.p.q=0

• Usually represented as Q• By definition QQt=I• Matrices that rotate coordinate axes are

orthogonal matrices

1 1 1

2 2 2

n n n

q p rq p r

q p r

"#

# # $ #"

Rotation in 2D and 3D• Rotation through an angle θ

• Rotation + translation

• Rotation in 3D – φ about z axis, θ about new x axis,

ψ about new y axis.

θ

y

x

x’

y’

'

'

cos sinsin cos

xxyy

θ θθ θ

= −

' '1 1

' '2 2

cos sin cos sinsin cos sin cos

t x pxx xt y pyy y

θ θ θ θθ θ θ θ

+ = + = +− −

' cos cos cos sin sin cos sin cos cos sin sin sin' sin cos cos sin cos sin sin cos cos cos cos sin' sin sin sin cos cos

x xy yz z

ψ φ θ φ ψ ψ φ θ φ ψ ψ θψ φ θ φ ψ ψ φ θ φ ψ ψ θ

θ φ θ φ θ

− + = − − − + −

Rotation matrix• Rotates a vector represented in one

orthogonal coordinate system into a vector in another coordinate system.– Since length of vector should not change||Qx||=||x|| for all x– Since Q will not change a vector along

coordinate directions QQt = I– Columns of Q are its eigenvectors.– Eigenvalues are all 1.

Similarity Transforms• Transforms vector represented in one basis to vector in

another basis• Let X={x1,…,xn} and Y={y1,…,yn} be two bases in a n

dimensional space– There exists a transformation A which takes a vector expressed in

X to one expressed in Y, – Inverse transformation A-1 from Y to X also exists.

• Let B and C be two matrices. Then ifC=A-1B A

B and C represent the same matrix transformation with respect to different bases and are called Similar Matrices.

• If A is orthogonal then C=AtBA

andi i i i i ij jAα β β= = =u x u y x

Eigenvalue problem

Remarks: Eigenvalues and Eigenvectors• Eigenvalues and Eigenvectors of a real symmetric matrix are real.• In general since eigenvalues are determined by solving a

polynomial equation, they can be complex.• Further roots can be repeated ! multiple eigenvectors correspond

to a single eigenvalue.• Transforming matrix into eigenbasis yields a diagonal matrix.

QtAQ=ΛΛΛΛ ΛΛΛΛ is a matrix of eigenvalues– Knowing the eigenvectors we can solve an equation Ax=b. Rewrite

it as QtAQQtx=Qtb ΛΛΛΛy=f

– Where y=Qtx and f= Qtb – Can get x from y x=(Qt)-1y = Qy

• Determinant is unchanged by an orthogonal transformation. • Determinant: Det(A) = λ1 λ2 …λn

When is Ax=b Solvable?• When does the equation Ax=b have a solution?

– Usual answer is if A is invertible– However in many situations where A is singular there still may

be a meaningful solution.

• Fredholm Alternative Theorem.– Look at the homogeneous systems

Ax=0 (1) A*y=0 (2)– If (1) has only the trivial solution then so does (2). This occurs

only if det(A) ≠ 0 (if A is invertible).Then Ax=b has a unique solution x=A-1b

– If (1) has nontrivial solutions then det(A)=0.• This means rows of A have interdependencies. In this case b must

reflect those dependencies

• If 2nd row of A is a sum of the 1st and 3rd rows, then b2=b1+b3

• If there are k independent solutions to equation (1) then A has a kdimensional nullspace.

• A* also has a k dimensional nullspace (but with different solutions). – Let these solutions be n*1, n*2, …, n*k

• For Ax=b can have solutions iff<b,n*j>=0 j = 1,…,k

• b must be orthogonal to the nullspace of A*.

• Any solution with y2=-y1 and y3=y1 satisfies the adjoint equationor the nullspace of A* is α [1,-1,1]t

• Here <b,n*1>= -1(≠0). So equation has no solution. • However if b=[1, 2, 1]t we would have a solution• General solution is x = x~+ckn*k where x~ is a particular solution.

1 1

2 2

3 3

1 1 1 1 1 2 1 02 1 1 3 or 1 1 2 01 2 0 1 1 1 0 0

x yx yx y

− = = − − =

−

Ax b

Least Squares• Number of equations and unknowns may not match• Look for solution by maximizing ||Ax - b||• (Aijxj-bi).(Aikxk-bi) with respect to xl• Recall

• Same as the solution of AtAx=Atb• Shows the power of the index notation

– See again the appearance of AtA

( )

( - ) ( - ) =0

( ) ( - ) ( - ) ( ) 0

( - ) ( - ) 2 0

ij j i ik k il

ij jl ik k i ij j i ik kl

il ik k i ij j i il il ik k il i

il ik k il i

A x b A x bxA A x b A x b A

A A x b A x b A A A x A bA A x A b

δ δ

∂∂

+ =

+ = − =

=

%

% %

% %

iil

l

xx

δ∂ =∂

Singular Value Decomposition• Chief tool for dealing with m by n systems and singular

systems.• Singular values: Non negative square roots of the

eigenvalues of AtA. Denoted σi, i=1,…,n– AtA is symmetric " eigenvalues and singular values are real.

• SVD: If A is a real m by n matrix then there exist orthogonal matrices U (∈!m×m) and V (∈!n×n) such that UtAV= Σ =diag(σ1, σ2,…, σp) p=min{m,n}

A= U Σ Vt

• Geometrically, singular values are the lengths of the hyperellipsoid defined by E={Ax: ||x||2=1}

• Singular values arranged in decreasing order.

Properties of the SVD• Suppose we know the singular values of A and we know

r are non zeroσ1≥ σ2 ≥ … ≥ σr ≥ σr+1 = … = σp=0

– Rank(A) = r. – Null(A) = span{vr+1,…,vn}– Range(A)=span{u1,…,ur}

• ||A||F2= σ1

2+ σ22+…+ σp

2 ||A||2= σ1

• Numerical rank: If k singular values of A are larger than a given number ε. Then the ε rank of A is k.

• Distance of a matrix of rank n from being a matrix of rank k = σk+1

Why is it useful?• Square matrix may be singular due to round-off errors.

Can compute a “regularized” solution– x=A-1b=(U Σ Vt )-1b =

• If σi is small (vanishes) the solution “blows up”• Given a tolerance ε we can determine a solution that is

“closest” to the solution of the original equation, but that does not “blow up”

• Least squares solution is the x that satisfies AtAx=Atb

• can be effectively solved using SVD

1

tni

ii iσ=∑ u bv

11

,tki

r i k ki i

σ ε σ εσ +

=

= > ≤∑ u bx v